This invention relates to the field of telecommunications in more particularly to a full duplex PCM modem system in which upstream channel impairments or variations are measured during a period of silence within the regular rate renegotiation sequence.
In a full duplex modem system having interconnected analog and digital modems, after the initial startup procedure and when both modems have commenced data transmission, there may come a time when the digital modem detects or senses the need to update the analog modem parameters. This is done by re-probing the upstream channel to ascertain if the error rate has increased. Since the initial startup has been completed, the analog modem transmission is locked to the network timing via a loopback timing scheme. This lock to network timing is important because loopback timing is an essential requirement to correctly estimate the channel.
In order to get the loopback timing the modems must be in the full duplex mode. It is however, at this point that echo becomes an issue. When in the full duplex mode, the modem transmitters are keyed on and there exists an echo path from the modem""s transmitter, through digital-to-analog and analog-to-digital converters to the modem""s receiver where the channel probing takes place. Although echo cancellation at digital modem can be quite effective, it is never 100% effective.
More specifically, it is important to be able to generate a series of parameters at the digital modem which are utilized by the analog modem to reconfigure itself to take into account channel variations. It is these channel variations or impairments which are to be measured at the digital modem. In some instances, pre-equalization parameters, transmit constellation, and transmit power at the analog modem may need to be changed to accommodate changes in the upstream communications channel. These changes are made at the analog side in order to be able to maximize the performance of the analog modem, given the channel variations sensed at the digital side.
In full duplex PCM modem systems, echo canceling is utilized both in the analog modem and the digital modem to prevent the echo generated by modem transmitters from interfering with channel measurements or the data transmission itself. Echo cancellation has been effective in the past to preclude many types of distortions. However, it is only with difficulty that echo cancellers can be of sufficient quality to cancel all of the transmitted energy which is usually reflected as an echo back into the modem""s receiver. This is especially troublesome when measurements are to be taken to measure channel variation or impairments during full duplex modem operation.
In order that the digital modem be able to measure the upstream channel characteristics, a test or training protocol is transmitted from the analog modem to the digital modem. The test or training protocol has known characteristics which are altered by the channel and are detected at the digital modem so that the effect of the channel can be ascertained. Upon ascertaining channel variations or impairments, a set of parameters is transmitted back to the analog modem to reconfigure or reset the analog modem to optimize it for the particular channel characteristics sensed. Typical parameters are the mapping parameters, constellation sets, and pre-equalization parameters.
It is known that full duplex transmission results in an echo path from the transmitter through the digital-to-analog converter at the central office or CO, and back through the analog-to-digital converter to the receiver where the channel characteristics are measured. As will be appreciated, an inability to adequately cancel the effects of the echo on the received signal results in the missmeasurement of the channel and therefore a sub-optimal set of parameters being transmitted from the digital modem back to the analog modem.
Rather than operating the digital and analog modems in full duplex mode, in the subject invention in the rate renegotiation sequence the transmission from the digital modem is inhibited during a window in which the test or training protocol from the analog side is transmitted. The cessation of the transmission at the digital modem completely eliminates echo as a factor at the digital modem""s receiver, such that the test or training protocol signals can be used to accurately measure the upstream channel. Note that the silence period is limited, because if prolonged, loopback timing will be lost. If loopback timing is lost, the channel cannot be correctly estimated.
With echo-free measurement of the channel characteristics, the parameters sent or transmitted back to the analog modem more accurately reflect the condition of the channel, thus resulting in a more accurate setting of the precompensation elements utilized in the analog modem. Precompensation in general includes the setting of pre-equalization constants, the mapping parameters to set the output power of the analog modem and the transmit constellation to optimize the constellation for the transmission of the data.
With an optimal set of parameters it will be appreciated that communication is made more robust and that the speed at which the system can operate is increased. Were this not the case, as is common with high error rates engendered by changing channel conditions, speed must be reduced or throttled back.
In summary, in a full duplex PCM modem system, a method and apparatus is provided for robust measuring of the communications channel in which the digital modem""s transmitter is silenced during generation of training signals by the analog modem, with the silencing of the a, digital modem""s transmitter eliminating problems associated with echo during channel measurement at the digital modem. The above method thus eliminates echo during channel estimation and removes any dependencies on the performance of the echo canceller normally used.